Instructors can view previously sent email content

Previously on the send email page of the instructor dashboard, instructors could only view task information about emails they've sent for their course in the past.
In addition to this, I've now added the ability to see the content of all previously sent emails. A "show email content history" button has been added to the page. When clicked, a table displaying the subject line, number of emails sent, and date/time of submission for each previously sent email is created. An instructor can then click on any subject line to see the content of that email, displayed in a modal window that appears on the page. The window is also equipped with a "copy email body to editor" button, which copies the emails contents to the tinyMCE editor, so that an instructor can easily resend an email that they've sent in the past.

This PR addresses some aspects of this ticket: https://openedx.atlassian.net/browse/LMS-10345
Or even to the `email-background` class to be more specific, actually.